The invention relates to a speech analysis apparatus for measuring and analyzing vocal tremor. Sustained phonation from a subject contains information in a composite form of vocal stability. Sustained phonation can be demolated into low frequency amplitude and frequency modulation components of the fundamental frequency. These components which make up vocal tremor occur in the range of 1 to 25 Hz. In demolation of sustained phonation the relationship between fundamental (carrier) frequency and modulation frequency is important. The factor of separation must be large enough to give proper definition to the modulating signal. As the separation factor decreases demodulation level will decrease, even though the modulation level has not changed, thus yielding a demodulation level that is erroneous. Demodulation of frequencies near the carrier frequency are useful for cycle to cycle, jitter and shimmer analysis, however they are not useful for tremor analysis. Once the fundamental frequency has been accurately demodulated the levels of modulation serve a useful purpose. Low levels of modulation are associated with normal phonation, while high levels of modulation are associated with pathological phonation. Different ranges of demodulated frequencies are associated with different physiological sources. Therefore, it is usefull to measure the level and frequency of these components. Different combinations of amplitude and frequency modulation frequency and level could be used to isolate different sources of modulation in the vocal tract. For instance, the primary source for amplitude modulation is subglottal air pressure, for frequency modulation neromuscular control of the cricothyroid muscle. Simultaneious display of these parameters; amplitude modulation frequency, amplitude modulation level, frequency modulation frequency, and frequency modulation level are useful to indicate a change in tremor which can occur in one, or a combination of parameters. Vocal tremor may be an early symptom or only symptom of a neurologic disease. The frequency and level of vocal tremor may differ among diseases of different neural subsystems. Therefore, analysis of vocal tremor with the invention may be used to make important contributions to early and differential diagnosis of neurological diseases and consequently to treatment decisions.
Earlier devices have used demodulation of vocal signals in the amplitude or frequency domain for other applications. One device produces frequency demodulation up to cycle to cycle frequencies of the fundamental frequency. A microphone signal is preconditioned with various user selected preset filters. After filtering, the signal is converted to a square wave of the same period as the fundamental frequency. This square wave drives a frequency counter, indicating fundamental frequency, and frequency to voltage converter which provides an arbitrary frequency demodulation. This signal is used to drive the vertical input to an oscilliscope display.
Another device produces amplitude demodulation and utterance decay on connected speech for use in suicidal predisposition analysis. A microphone signal of several utterances is converted to a digital format and signal components below 200 Hz and above 10,000 Hz are removed. The signal is further conditioned and produces a waveform which depicts onset and offset, as well as amplitude modulation of the utterances. By determining high or low levels of amplitude modulation and rate of decay of the utterances, a subject is analyzed for suicidal predisposition.
Although other methods of demodulation have been presented, none provide means for conditioning the input signal and subsequent analysis of the fundamental frequency signal as presented here. It is an object of the invention to provide a calibrated real time device for accurate level and frequency measurement of amplitude and frequency modulation components in sustained phonation. These measures of stability can assist in diagnosis of nerological diseases, and quantify the effects of pre and post thearapy, surgery, or medication. Another object of the invention is to provide amplitude and frequency demodulated outputs allowing external analysis of vocal tremor components.